1. Field of the Disclosure
This disclosure relates generally to envelope tracking power amplifier systems, and more specifically to envelope tracking power amplifier systems with improved characteristics.
2. Description of Related Art
Envelope Tracking (ET) systems may be found in the radio frequency (RF) transmitter section of a radio where power efficiency is important, such as in cellular radios used in mobile phones. A typical ET system includes a variable power supply supplying a power amplifier (PA) with a dynamically changing supply voltage that tracks the amplitude of the modulation. The goal of such an ET system is to improve power efficiency by operating the PA with low headroom.
The supply voltage level can be determined with a look up table that references amplitudes to values for the supply voltage. FIG. 7 illustrates the nature of choices available for a lookup table. As an example, for an amplitude corresponding to PA output power of 22 dBm, the lookup table may contain PA supply voltage values ranging from 1.2V to 5V. The choice of this value may be set to provide a good balance between PA efficiency and linearity. If the PA supply voltage is set too low, the PA operates with lower head room, and thus with higher efficiency, but with higher distortion. Conversely, if the PA supply voltage is set too high, the PA operates with higher head room, and thus with lower efficiency, but the additional headroom allows for lower distortion levels.
The values for the lookup table are typically determined in a factory through characterization on a typical PA on a typical radio device under typical conditions. This initial set of values is then copied into other ET systems. However, during actual operation of the other ET systems, the PA may exhibit different characteristics from those during characterization of the typical PA, depending on factors such as PA process and manufacturing tolerances, power supply circuit variations, environmental factors, temperature, operating frequency modulation formats and antenna mismatch. Thus, the initial set of values may not operate the PA with the right balance of power efficiency and distortion.